The early detection of invading viruses by the host depends on a limited number of specific intracellular receptors that detect viral patterns and activate signal transduction cascades, thereby triggering interferon (IFN)-mediated anti-viral defense mechanisms. Key virus-detecting receptors include the nucleic acid recognizing Toll-like receptors and the cytosolic RNA receptors RIG-I and MDA-5. Specifically, RIG-I has emerged as a key receptor in sensing emerging viruses, including the influenza virus and hepatitis virus C (HCV), whereas MDA5 responds to the infection of picornaviruses and noroviruses. Members of the tripartite motif (TRIM) protein family also play major roles in the inhibition of the lifecycles of viruses. On the other hand, the efficacy of the IFN responses has led to a number of viruses evolving various evasion strategies against the host IFN system. HCV and influenza A virus are emerging pathogens associated with severe liver disease and fatal respiratory diseases, respectively. Immune evasion strategies of influenza virus and HCV are essential to escape the host anti-viral response, allowing chronic or sustained viral replication in vivo and resulting in severe and sometimes deadly diseases. This multi-project Cooperative Agreement (U19) grant application will attempt to address how the host develops firsthand innate immune recognition, how it is activated in response to viral infections, and how viruses have evolved the multiple mechanisms necessary to thwart the host innate defense. This application consists of five projects with multidisciplinary schemes. Project 1 utilizes cell biological and immunological approaches to study RIG-I and TRIM25 mediated immune surveillance against virus;Project 2 focuses on the structural basis of RIG-I and MDA-5 mediated anti-viral response;Project 3 uses single molecule fluorescence techniques in vitro and super-resolution imaging of cells to dissect the RIG-I pathway;Project 4 investigates how HCV interacts with host cells to evade intracellular innate immunity;and Project 5 studies how the influenza A virus NS1 protein prevents cytoplasmic viral sensors from recognizing viral infections, us, the goal of this U19 project is to unravel the key molecular events of host immune recognition and viral immune escape, which ultimately give insights for novel antiviral therapeutic interventions and safe and effective vaccine development.